Underbalanced drilling is advantageous in many circumstances. Underbalanced drilling generally involves the practice of drilling with anticipated downhole pressure greater than hydrostatic pressure of the mud column. Formation pressure is not sufficiently contained or controlled by drilling fluids to prevent flow from the formation. Formation flow could potentially reach the surface to blow out the well if downhole pressure were great enough but for the surface pressure control systems that are used to control well pressure. The rotating blow-out preventer allows the operator to seal around the drill pipe and continue drilling even when the well pressure at the surface is greater than atmospheric pressure.
In horizontal well drilling as compared to vertical well drilling, it may be more difficult to establish well control by hydrostatic fluid head due at least in part to the slower build-up of hydrostatic pressure with well depth (which is not vertical depth) as compared with the build-up that normally occurs rapidly with well depth when drilling vertically oriented wells. The well control problems caused by lack of hydrostatic pressure may be made worse by hole conditions such as abnormal pressures, formation seepage, and lost circulation. In some cases, operators have saved hundreds of thousands of dollars in drilling fluid costs alone by drilling horizontal wells underbalanced. The safety of the operation may also be improved by this method because of the additional pressure control capability of the rotating blow-out preventer used for underbalanced drilling pressure control purposes. While drilling with the rotating blow-out preventer, sudden changes in hole conditions do not result in a dangerous blowout condition that may sometimes not be detected in sufficient time to effectively close in the well. For instance, drilling into a lost circulation zone whereby hydrostatic pressure may be reduced due to fluid loss could result in a sudden loss of pressure control. However, the seal in the rotating blow-out preventer quickly and automatically increases around the drill pipe to account for such sudden changes. In drilling vertical wells, the rotating blow-out preventer may be useful as an additional safety control device because similar fluid loss conditions may also result in well pressure control problems that could be easily handled by use of a rotating blow-out preventer.
Another significant advantage of underbalanced drilling, in either vertical or horizontal wells, is the avoidance of formation damage caused by overbalanced drilling fluids. Repair of formation damage caused by overbalanced drilling may be difficult, time consuming, and limited. Thus, formation damage may significantly reduce a well's ability to produce, thereby significantly affecting profitability of the well.
Another advantage of underbalanced drilling is the result of greatly increased accuracy of logging tools and other measurement devices. Formation invasion by drilling fluid is perhaps the greatest cause of inaccuracies in well logs. For instance, to obtain good measurements of uninvaded formation characteristics, logging tools are expected to compensate for mud cake build-up of the drilling fluid in the borehole, a flushed zone around the borehole wherein all moveable formation fluids have been flushed therefrom, and a partially flushed zone around the borehole wherein moveable formation fluids have been partially flushed therefrom by a not necessarily evenly decreasing percentage until non-invaded formation is reached. It will be understood that compensation techniques, while very useful, cannot always compensate for and accurately determine the characteristics of the non-invaded formation. Therefore, significant zones of oil or gas may remain undetected, or have distorted readings, that cause valuable production zones to be passed over when the operator reviews and selects what may incorrectly appear to be the best producing zones. In the absence of invasion of drilling fluid into the formation due to underbalanced or even near balanced drilling, the accuracy of logging tools is greatly increased because the formation is not invaded and the formation fluids present themselves somewhat more naturally the borehole. This means that the operator has more accurate information with which to make decisions. Other well measurement tools, such as coring tools, will also produce more accurate readings. Thus, there are many advantages to underbalanced drilling.
For underbalanced drilling, the rotating blowout preventer is mounted to the top of a stack of conventional BOP's and can control surface back pressure in a range depending on the rotating blow-out preventer pressure rating. The well is drilled with an underbalanced fluid, such as diesel, water mixed with nitrogen, air, gas, or the like. The rotating blow-out preventer allows rotating and stripping of the drill string during the drilling operation, a significant advantage that normal BOP's do not provide.
Because the rotating blow-out preventer is typically mounted on top of a conventional BOP stack, the length or height of the rotating blow-out preventer is often important depending on the rig set up. Space between the conventional BOP stack and the rotary table and/or drill floor may be strictly limited by the size of the drilling rig and the depth of the cellar to a length required to manipulate the largest drill stands it can drill with. Thus, for general purpose use with many drilling rigs, it is highly desirable for the rotating blow-out preventer to be limited in height. As a result of height restraints, the length of sealing area is limited and must still safely seal variable sized drill pipe, drill pipe connections, and the square or hexagonal kelly, if present for rotary table drilling operations. For purposes of the present application, it assumed that the word tubular defines drill pipes, kellys, and so forth.
The rotating blow-out preventer may use hydraulically activated packing elements mounted for rotation with the drill pipe. If the packing elements are large and heavy, then the bearings may wear more rapidly. Large packing elements and large bearings are quite time consuming to change out, if it becomes necessary to make a replacement. In some rotating blow-out preventer's, the entire top of the rotating blow-out preventer housing must be removed before the bearings can be changed. This may also require removal of the driller's rotary table, which may also be time consuming and may often require a competent rig mechanic to be present.
Large packing elements may not be flexible enough to seal with all drilling elements, such as square or hex-shaped kellys, thereby requiring an additional kelly packing device that adds additional complexity to operation and cost of the Rotating blow-out preventer. Most rotating blow-out preventer's have some provision for changing out at least the most wearable parts of the drill pipe packing elements without the need to remove the rotating drill table. Generally, the packing element, or the most wearable portion thereof, is retrievable through the hole in the drill table. In some designs, this requires fishing to secure the most wearable portion of the packing element. The least wearable portion of a dual element packer may not be available for replacement without extensive time to disassemble the rotating blow-out preventer. Designs for more quickly releasing the packing elements may include removable clamps that have to be manually released, as by a threaded bolt latch, and then manually detached from the rotating blow-out preventer housing. In some designs, hydraulic controls may release the clamp, but the clamp holding the packing elements within the rotating blow-out preventer must then be manually detached from the rotating blow-out preventer housing before the packing elements are removed. Such work with heavy moveable equipment within small enclosures can well be hazardous.
Another problem with presently existing rotating blow-out preventer's is the high failure rate of the upper bearing seal and/or upper bearing. Failure may occur due to the fact that most of the pressure drop between wellbore pressure and ambient pressure is across the upper bearing and seal. The upper and lower bearing seals must seal between a stationary element, such as the rotating blow-out preventer housing, and the rotating elements of the packing assembly. Typically, the pressure drop across the bottom seal and/or bottom bearing is a pressure drop of only about 250 psi or so, because hydraulic activating fluid is typically maintained in the range of from 0 to 500 psi above the well head pressure for activating the packing elements to seal against the drill pipes. However, the upper seal and/or upper bearing must then have the remainder of the pressure drop between the well head pressure and ambient pressure, which pressure depends on the rating of the rotating blow-out preventer and the well head pressure upon which it is used. The large pressure drop across the upper seal and/or bearing places a strain on the upper bearing elements and the upper seal that may cause earlier failure of such bearings. In rotating blow-out preventer systems where bearing change-out is a lengthy process, this is an especially significant problem due to excessive lost rig time caused by such an upper bearing and/or seal failure.
Consequently, an improved rotating blow-out preventer is desirable to provide accurate sealing over a wide range of profile variations in pipe and kellys, quick change-out not only of seals but also of bearings through the rotary table, and provisions to improve the lifetime of especially the upper rotary seals and bearing. Those skilled in the art will appreciate the present invention that addresses these and other problems.